1. Field of the Invention
The present invention relates to a positioning apparatus and, more specifically, relates to a positioning apparatus included in a photolithography apparatus used for producing devices such as semiconductors and liquid crystal devices.
2. Description of the Related Art
When using a photolithography apparatus for producing a device such as a semiconductor or a liquid crystal device, the substrate must be positioned highly accurately. A positioning mechanism for the photolithography apparatus may include a coarse movement stage movable in a large area and a fine movement stage movable in a smaller area as compared to the coarse movement stage.
Japanese Patent Laid-Open No. 2001-143997 discloses a method for measuring the displacement of a fine movement stage, on which a wafer is disposed, in the Z direction (vertical direction) for a photolithography apparatus. This method will be briefly described with reference to FIG. 6.
The positioning apparatus illustrated in FIG. 6 includes a coarse movement stage Sc and a fine movement stage Sf. An X-direction moving element Mx of the coarse movement stage Sc is movable in the X and Y directions relative to a stage base Bs. The fine movement stage Sf is disposed on the X-direction moving element Mx and is movable in a smaller area relative to the X-direction moving element Mx.
An instrument base Bi is provided for mounting components on the measurement reference side of the positioning apparatus. The instrument base Bi includes an X interferometer 11 for measuring the position of the fine movement stage Sf in the X direction and a Z interferometer 12 for measuring the position of the fine movement stage Sf in the Z direction.
A mirror surface p1 for measuring the position in the X direction and a bar mirror 54 and a mirror surface p2 for measuring the position in the Z direction are disposed on the fine movement stage Sf. A mirror surface p7 for measuring the position in the Z direction is disposed on the instrument base Bi.
A measurement light beam emitted from the Z interferometer 12 is reflected at the mirror surfaces p2 and p7 and is returned to the Z interferometer 12. The measurement light beam is interfered with a reference light beam (not shown in the drawing) to measure a displacement dx+dz. A measurement light beam emitted from the X interferometer 11 is reflected at the mirror surface p1 and returned to the X interferometer 11. The measurement light beam is interfered with a reference light beam (not shown in the drawing) to measure a displacement dx. The displacement in the Z direction is measured by obtaining the difference between the displacement dx+dz and the displacement dx.
In the above-described structure, the reference light beam is guided for a predetermined distance. Therefore, if there is a temperature difference between the light paths of the measurement light beam and the reference light beam, an error occurs in the measured value. In particular, when determining the displacement in the Z direction using the X and Z interferometers 11 and 12, the X and Z interferometers 11 and 12 are both affected by the temperature fluctuation in the environment, causing a twofold error to decrease the measurement accuracy. In addition to the error due to the temperature difference of the light paths, errors due to other reasons will also be twofold. Furthermore, depending on the position of the stage, the distances between the X and Z interferometers 11 and 12 and the mirror surfaces p1 and p1 greatly change. As a result, the measurement accuracy is significantly reduced.
It is undesirable to dispose a mirror having a reflective surface at an acute angle relative to the vertical direction, such as the bar mirror 54, as illustrated in FIG. 6, on the fine movement stage Sf because it will have negative effects on the weight and rigidity of the fine movement stage Sf and will cause a degradation in the control of the fine movement stage Sf.